The invention relates to a process for separating impurities from a suspension by flotation, covering aeration of a liquid with gas until it is saturated, followed by expansion of the liquid saturated with gas, during which gas bubbles form, as well as a device for separating impurities from a suspension by flotation, covering at least devices for aerating a liquid with gas until the liquid is saturated, for then expanding the liquid saturated with gas, during which gas bubbles form, and for adding the liquid with gas bubbles to the suspension and feeding it to a flotation cell, as well as a device for generating gas bubbles in a liquid saturated with gas.
Froth flotation is a physical process for removing impurities from suspensions. This process requires gas bubbles to be generated in a suitable quantity and size distribution. Hydrophobic or waterproofed substances are carried to the surface of the liquid by the gas bubbles adhering to them and can be removed there together with the froth. Processes of this kind are known, for example, from DE 41 16 916 C2, and have reached a high technical standard. Self-priming injectors are often used to generate the gas bubbles and mix them into the suspension. Here, the flow of suspension charged with impurities exiting from a nozzle generates negative pressure and sucks in gas, which is mixed with the suspension as a result of the pulse exchange between gas and liquid. A device of this type is described, for example, in DE 34 12 431 A1.
The DE 198 45 536 describes a de-inking process where flotation bubbles are formed in a suspension saturated with gas and subsequent expansion, where sticking fibers are separated in a following step. In this process pressures of 0.6 to 1.2, bars are used and the size of the created bubbles can hardly be regulated.
A process is also known in which pressurized water, saturated with gas, is injected into a flotation cell in order to thus cause gas bubbles to form. WO 90/10502 A1 describes a nozzle provided for this purpose, where the flow passages and directions can be modified, allowing the nozzle to be cleaned easily.
The aim of the present invention was to provide a flotation process and devices for use therein, which offer improved conditions for setting the optimum size and quantity of bubbles in the suspension and thus, make the process more effective.
In order to achieve this aim, the invention provides for the liquid saturated with gas expanding separately and before the liquid containing the gas bubbles is added to the suspension with the impurities, as part of the process described above. Thus, formation of gas bubbles can be uncoupled from feed into the flotation cell and the conditions prevailing there, and can be optimized independently thereof.
A further improvement can be achieved in an advantageous configuration of this process if the liquid is also expanded separately after it has been aerated with gas.
In a tried-and-tested method, the liquid can expand by injecting the liquid saturated with gas.
According to a further feature of the invention, it is advantageous if the liquid is expanded by a propulsion jet of the liquid saturated with gas being sprayed into an enclosed space, where the stream diverges. In this way, the pressure differential needed to release the gas bubbles can be generated easily, but still very effectively.
In order to achieve the aim of the present invention, the plant for removing impurities from suspensions with the aid of flotation is also characterized according to the invention by the device for expansion of the liquid being arranged separately from the device for adding the liquid with gas bubbles to the suspension and separately from the flotation cell.
In order to provide an easy method of adjusting the pressure loss during expansion and thus optimise generating of the gas bubbles for the application in question, as well as optimise the given or desired conditions, a further feature of the invention provides for a nozzle with adjustable cross-section, preferably also an adjustable cross-section area. A device of this type can be used in all flotation plants and cells, even if the liquid containing the gas bubbles, particularly the suspension itself, is injected directly into the flotation cell.
It is an advantage here if the nozzle has an annular opening, according to a preferred configuration.
A particularly good flow characteristic and spreading of the liquid jet exiting from the nozzle is obtained in a configuration where the outer limit of the nozzle has the shape of a Laval nozzle.
It is also an advantage if the nozzle with annular opening has a central section formed by a cone converging into the enclosed area, which contributes to the favourable flow characteristics after discharge of the liquid jet.
If, according to a further feature of the invention, the cone can be moved in axial direction and/or rotated round its own axis, the cross-section area of the opening can be modified easily and thus, the pressure loss obtainable can be set and the propulsion jet nozzle can be cleaned or flushed out in the event of clogging.
In addition, by improving the flow characteristics, optimum gas bubble formation is assisted by the device being designed such that the enclosed area is formed by a pulse pipe with a largely constant cross-section and, preferably, an adjoining diffuser whose cross-section increases from the nozzle outwards.